Repeated surveys of short level lines in the Shumagin Islands, Alaska, reveal coherent tilt signals associated with subduction of the Pacific plate beneath the North American plate in the Shumagin seismic gap. Ten years of steady tilt down toward the trench is interrupted during 1978--1980 by a rapid episode of reverse tilt. The ''normal tilt represents surface deformation as subduction occurs with the plate boundary locked to at least 60 km depth. Using all available tilt, sea level, and seismic data, we interpret the tilt reversal as due to an episodic reverse slip of ≈80 cm magnitude on the plate boundary between ≈70 km and ≈20 km depth, downdip from the main seismogenic zone, which remains locked. This event causes an increase of stress on the locked main thrust zone. We speculate that such events may be a regular process at subduction zones, that great plate boundary earthquakes may be more common during their occurrence, and that their onset may be detectable early enough to give warning of an increase in probability for the occurrence of a great earthquake.